The present invention relates to video production switchers, and more particularly to a motion detector and key signal interpolator using the same for a chroma keyer.
There are many examples of motion detection circuits currently available. One such traditional motion detector, such as the motion detector used in the Kaleidoscope Digital Picture Manipulator, manufactured by The Grass Valley Group, Inc. of Grass Valley, Calif., United States of America, is shown in FIG. 1. Detection is performed by a pixel-by-pixel absolute difference between two frames with the same field polarity, i.e., inter-frame motion. A video signal is input to a series of three field delay circuits to provide four outputs representing four consecutive fields. The odd fields from two consecutive frames are input to an inter-frame motion detector, and the even fields from the same frames are input to another inter-frame motion detector. If the difference between the two fields is less than a threshold value, then no motion is detected; and if the difference is greater than the threshold value, motion is detected. The outputs from the inter-frame detectors are input to a programmable read only memory (PROM) which provides a motion value. A problem arises with this motion detector when an image is being resized that has objects at regular intervals moving at a constant horizontal rate that matches the field interval, i.e., 60 Hz for 525 line D1 format, such as characters generated by a character generator. When a group of characters moves horizontally at a rate such that a character is superimposed by another character in succeeding frames, the motion detector is "fooled" into indicating no motion since the difference between the two frames is less than the threshold value. However when interpolated using both fields of a frame, the result is an incorrect interpolation between the character and the background. Where as a result of the motion the character exists only in one of two consecutive fields, then subsequent interpolation produces a combination of black and white pixels to produce a gray pixel output.
What is desired is to be able to detect motion when horizontal objects in a video signal are moving horizontally at a field rate.